The strong paramagnetic property of oxygen compared to other gases has been used, as well known, for measurement of oxygen contents of gas mixtures. Expecially when measuring oxygen concentration of respiration gases it would be important to reach a rapid enough (approximately 100 ms) response time to follow instantaneous concentration values. The measurement methods exploiting paramagnetism of oxygen have been based on either thermomagnetic or so called Pauling principle, which however have some drawbacks like slowness resulting from volume requirements of the measuring cell or dependence on gas flow rate.
A quick response and flow independent method based on paramagnetism of oxygen has been presented in patents DT No. 164894 and U.S. Pat. No. 3,584,499 H. Hummel. In a device according to these patents sample and reference gas are conducted to a measurement cell placed in a slot between magnet poles in such a way that the two gases will be mixed in the homogenous magnetic field and the mixed gas will be pumped out through a single conduit. Magnetic field is chopped with a proper frequency and the AC-pressure difference signal proportional to the difference of oxygen partial pressure of sample and reference gas is measured with microphone connected between gas input conduits.
Because of some drawbacks associated with construction of measurement cell, development of stable enough and noise insensitive oxygen analyser based on these patents has not been totally successful. Especially the problems seem to arise from the measurement cell, where the measurement space has been built by a separate spacer plate made of antimagnetic material and placed into the air gap of magnet. In the patent the argument for need of spacer plate is minimizing of magnetomechanical interference, but in practice sealing materials needed to ensure tightness of the construction lead to less rigid structure and in addition glueing or other bonding is difficult to make so that extra glue or bonding material will not be spread to the cell or gas conduits.